1. Field of the Invention
The present invention relates to a simple discharge detector for a power equipment using the acoustic emission technique, in which acoustic emission signals are particularly transformed into square-wave signals by a comparator and then transmitted to a microprocessor, and a counter inside the microprocessor is provided to count times of partial discharge so as to determine whether the power equipment needs to be maintained or not.
2. Related Prior Arts
To promote efficiency of electricity transmission, voltage during transmission is greatly increased, and thus insulated material used for transformers and gas insulated switchgears (GIS) in plants are necessarily capable of standing high potential. When the insulated material worsens, the transformers may be damaged or even explode, which will result in power failure and disrupture of plants. Therefore, it's more and more important to maintain and detect power equipment.
So far, the technologies applied to detecting partial discharge of power equipment can be mainly classified into two kinds, namely electric method and non-electric method.
1. Electric Detection for Partial Discharge
In this method, pulse current of partial discharge in the detection circuit can be detected, which is easily quantified and highly sensitive. The detection circuit comprises either a couple capacitor or a Rogowski coil.
(1) Couple Capacitor
The couple capacitor with properties of divided potential and filtering is connected to the high-voltage side of the power equipment; and the pulse current signals of partial discharge are detected according to the principle of divided potential and filtering. As frequencies of the signals are generally more than 5 MHz, cost for computer treatment at post-end of the couple capacitor is quite high. Therefore, this method is normally applied to quality assurance of equipment before releasing from plants other than on-line detection.
(2) Rogowski Coil
Rogowski coil is generally applied to on-line detection. When partial discharge occurs, pulse current of partial discharge will flow through ground line of the power equipment, and the Rogowski coil may sense the pulse current from the ground line. However, frequencies of such current signals are as high as several MHz˜tens of MHz, and thus cost for computer treatment at the post-end is even higher. For common industries, grounding is a complicated issue as the partial discharge signals are easily interfered and covered by other signals. To solve such problems, a better filter or a more complicated filtering program is required and thus cost will be much higher.
2. Non-electric detection for partial discharge
The non-electric detection for partial discharge comprises acoustic detection and optical detection.
(1) Acoustic Detection
(a) Ultrasonic Microphone
When the source of partial discharge exists in the air, a phenomenon similar to corona occurs and ultrasonic waves generated from partial discharge will be transmitted via air. Therefore, the discharge source can be easily detected with an ultrasonic microphone. However, when the source of partial discharge is caused by media inside the equipment, for example, insulated oil in an oil-immersed transformer, resin in the cast resin transformer and SF6 in GIS; the acoustic waves of discharge are hardly transmitted to the air via theses media isolated from the air. Therefore, an acoustic emission sensor is required for detecting inside discharge.
(b) Acoustic Emission Sensor
The partial discharge occurring inside equipment is similar to pulses and will generate mechanical pressure waves inside the media. This phenomenon can be analogized to acoustic emission (AE), which is possibly caused by impact between molecules of interior material and adjacent structures. Such acoustic source will widely emit acoustic waves in the equipment, as shown in FIG. 1. Whether the acoustic waves will emit from the equipment to the air is determined by the acoustic impedance of these two media. In general, difference between them is too large to cause emission from equipment to the air. That is, it's unfeasible to detect an interior source with the ultrasonic microphone.
In the acoustic emission method, an AE sensor (81) firmly attached to the surface of equipment (82) is utilized, and mechanical pressure waves are converted into electrically signals by a piezoelectric material inside the AE sensor (81), as shown in FIG. 1. The AE signals are further amplified through a preamplifier (83). In this method, frequencies of the acoustic wave signals generally range from 20 kHz to 80 kHz, which are much lower than those obtained in the above methods and thus costs much less.
(2) Visual or Optical Detection
When corona discharge occurs in the power equipment, temperature on surfaces of the equipment will increase and thus the source can be detected with an infrared thermal radiometer. Alternatively, spectrum of light generated due to gas ionization in the discharge corona can be inspected with a UV discharge detector. However, facilities used for these two methods are expensive, and merely surface corona can be detected. As for discharge occurring inside or shielded, these facilities are ineffective.
(3) Analysis of Gas Dissolving in Oil
For the power equipment using insulated oil, partial discharge can be detected in certain situations by analyzing species and contents of gas generated during discharge and dissolved in the oil. As it usually takes a long time to dissolve the gas in the oil, therefore the oil is analyzed several times or continuously during operation of the equipment. However, analysis of the oil is usually performed with expensive and complicated instruments for a period of time, and results thereof require being precise enough. As application of this method is limited to the power equipment using insulated oil, therefore other methods have to be developed for GIS or cast resin transformers.
FIG. 2 indicates the principal for designing a general detector for partial discharge, in which a host (91) with high-speed CPU and a lot of memories and hard disks are required. Moreover, the analog/digital (A/D) converter (92) is designed multi-channeled, so that the analog signals in channels can be converted into digital data with time sharing. Huge data will be stored in memories with large volume and then analyzed and identify with the CPU. When finishing work with the CPU, these data are restored in the memories. As a result, it has to take minutes or more time to deal with data in each channel.
For general manufacturers, the desired function is just to find abnormal equipment other than analyze or identify data like a manufacturer or a maintainer of equipment.
In most plants, it's also difficult to detect equipment distributed in a large area. For example, cast resin transformers in a high-technology plant are generally arranged in a power control chamber with length and width more than ten meters. Moreover, GCB (gas circuit break) and GIL (gas insulated transmission line) are generally separated from a distance over hundreds of meters. For such situations, conventional detectors for partial discharge will be improper due to demerits such as signal decay of long cables and fencing of cast resin transformers in the high-technology plants.
In addition, lines for transmitting analogical signals can not be too long due to interference of signals generated by high-voltage equipment, and lines for transmitting digital data require superior anti-interference.
Accordingly, it's desired to develop a detector for partial discharge with better characteristics and lower cost.